This invention relates to ultrasound imaging systems and, more particularly, to methods and apparatus for imaging moving fluid and tissue.
Color Doppler, power Doppler and B-mode flow imaging are methods currently used to image blood flow. In each method, the flow signal is isolated from the larger tissue signal by a temporal high-pass filter which blocks out echoes from stationary or slowly moving reflectors. However, flash artifacts (also referred to as xe2x80x98flashxe2x80x99) occur as a result of tissue and/or transducer motion that obscures the desired flow signals.
Previous methods for ultrasound flash suppression involve one-dimensional processing along time (see, e.g., U.S. Pat. No. 5,782,769, issued Jul. 21, 1998) whereupon an abrupt change in flow brightness at a given spatial location is assumed to be an artifact and is suppressed. However, certain blood flow phenomena have similar temporal characteristics such that suppressing flash without suppressing the flow signal is difficult if not impossible. These prior art methods often include spatial smoothing as an adjunct to improve performance, but the primary flash suppression mechanism remains temporal filtering.
In flow imaging, enhanced flow discrimination is achieved by employing a robust and efficient method of suppressing flash artifacts through real-time processing of two-dimensional image frames so as to enhance visibility of the flow signals. The processing is performed on detected image data at the back end of the ultrasound imager prior to display. Real-time image processing at the back end of the ultrasound imager has become practical with the advent of faster and cheaper CPUs (central processing units) and digital signal processors.
The preferred embodiments of the invention take advantage of the spatial characteristic of flash artifacts to estimate and suppress them more effectively and reliably. Flash artifacts typically occur as regions of elevated signal strength (brightness or equivalent color) within an image. The flash suppression algorithm in accordance with a preferred embodiment comprises the steps of estimating the flash within an image and then suppressing the estimated flash. Here, the flash suppression mechanism is spatial filtering. An extension of this basic method uses information from adjacent frames to estimate the flash and/or to smooth the resulting image sequence. Temporal information from adjacent frames is used only as an adjunct to improve performance.